WEST COAST OF INDIA

SEDIMENTOLOGY AND GEOCHEMISTRY OF TIDAL MUDFLATS ALONG CENTRAL

WEST COAST OF INDIA

Ph.D. THESIS

BY

KSHETRIMAYUM TOMCHOU SINGH M.Sc.

DECEMBER, 2007

Professor and Head, Department of Marine Sciences,

Goa University, Taleigao Plateau,

Goa - India

DECEMBER, 2007

SEDIMENTOLOGY AND GEOCHEMISTRY OF TIDAL MUDFLATS ALONG CENTRAL

WEST COAST OF INDIA

THESIS

SUBMITTED TO THE GOA UNIVERSITY FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

IN MARINE SCIENCE

BY

KSHETRIMAYUM TOMCHOU SINGH M.Sc.

UNDER THE GUIDANCE OF

57$-'7 17

Dr. G. N. NAYAK ,.s IN e

DEDICATED TO

MY LATE GRAND PARENTS

i

STATEMENT

As required under the University ordinance OB.9.9 (iv), I state that the present thesis entitled "SEDIMENTOLOGY AND

GEOCHEMISTRY OF LIDAL MUDFLA TS ALONG CENTRAL WEST COAST OF INDIA" is my original contribution and the same has not been submitted on any previous occasion. To the best of my knowledge, the present study is the first comprehensive work of its kind from the area mentioned.

The literature related to the problem investigated has been cited. Due acknowledgements have been made wherever facilities and suggestions have been availed of.

Place: Goa, India Date: 04.12. 2007

/

\1,4.- •

Kshetrimayum Tom ou Singh

CERT/FICA-WE

is Z.s to certify that the thesis entit le d "SEDIMENTOLOGY AND GEOCHEMISTRY OF TIDAL MUDFLATS ALONG

CENTRAL WEST COAST OF INDIA" submitted by MR.

KSHETRIMAYUM TOMCHOU SINGH for the award of the

Degree of Doctor of (Philosophy in Marine Science is based on his original studies carried out by him under my supervision. ate thesis or any part thereof has not been previously submitted for any other degree or diploma in any universities or institutions.

Place ^: Goa 'University Date :04.12.2007

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Dr. yak

(Research Guide)

Professor of Marine Science .7-fead, Department of Marine Sciences

Goa 'University

Goa - India.

Acknau4dgements

Throughout my research career many people have supported me with advice, comments, and suggestions, and I could make this thesis into what it is now. I am indebted to all of them but I would like to thank a number of them in particular.

First andforemost, I express my deep sense of gratitude to my research guide, Prof. G. Y. Kayak Head, Department of Marine Sciences, Goa

(University, Goa, who has offered me the opportunity to become a Marine Geologist. With his constructive comments and encouragements he has guided me through the ups and downs of research and introduced me to scientific community. heartiest thanks for all his Cove, constant motivation and encouragement.

I wish to place on record my thanks to (Prof Dilip Deo6agkor, Vice Chancellor, Goa 'University and Prof R Desai, Dean of the faculty for their support and encouragement.

I am thankful to Dr. Xijiv Nigam and Dr. B. Wagendra Nath, Scientists, 9V70, Goa, India and Vice Chancellor's nominees in the Eaculty Wcsearch Committee (T), for their constructive criticism and encouragement throughout the course of work.

I am grateful to Or S. X Shetye, Director, .7 ■ 11-0,. Goa, India, for kindly allowing me to utilize the required facilities at the Institute and I thankDr. ast C. Woo Scientist, 91 110, for his kind help in getting the X - ray diffraction analysis and in computing the results. I am grateful to Dr. D. 11. cBoro le , GOD, 91170, for his help in getting nopb

analysis, and suggestion in interpreting the results.

I am thankful to Director, Indian Institute of Geomagnetism (IIG),

Mum6ai India, for providing me the required facilities at the Institute

and I especially thank Dr. Nathani Basavaiah, Associate Professor,

HG, New Tanver, IMumbai for his pleasant and productive advice and discussion in the field of rock magnetism.

I Wilk( like to record my sincere thanks to Or C. Krisfinaian, Wrsearch Co-ordinator,Ocean and Atmospheric Science and Technology Cell (OSTC), (Mangalore 'University, for getting XIS patterns required for clay mineral analysis.

Wy sincere gratitude to Shri &Ai& Wovindra, Director, National Centre for Antarctic and Oceanic Wesearch (NCA0V, Goa for giving me an opportunity to participate in the 25 14 expedition to Southern Ocean and Larsemann Mills of Antarctica during my research period.

Special thank goes to Dr. 11. Sudhakor, Scientist, .NCAOX for encouraging my research period and the expedition to 'Southern Ocean and Larsemann Miffs. of Antarctica' from 25 74 Jan to

April' 2006, which was an unforgettable experience.

I'm thankful to faculty members of Department of Warine Sciences, Goa 'University viz Dr. .7f. B. %tenon, Dr. S. Vpadhyay, Dr. M. V Ilatta, Dr. C. V. Vvonkor, Dr. Afta6 Can, for their encouragement.

I'm a lso thankful to the non-teaching staffs of Department of Warine Sciences viz lir. Ysiarayan, Mr. Atchut, lir. Ashok, firs.

Sanjana, flrs. Ilatifda for their support and encouragement. I would like to thank (Dr. ft. ft. Sangodkor, Wegistrar, Goa 'University and his subordinates for their kind administrative help.

I owe a lot to my research colleagues, Or Wojeev Saraswat, Awes& Zvi, Vinay, Santosh, Wr. Sanjay Wont; fir.

Sanmukha, fls. Nutan, fls. (Deepti, 8s.Wocheat; flrs. Amita, fls

Sweety, fls ShilPa, JKrs. Sujata, lirs.gWani and fls. Linsy, who were directly or indirectly involved in this thesis with their constant encouragement and more importantly, made my stay at Goa an unforgettable one.

I greatly acknowledge the kind help, good cooperation, and social

alas, Faculty members of govt. College of Arts and Science, 7(grwar, during my field work at 70rwar and Gokorn. I gratefully acknowledge the assistance of several persons who played a significant role in sample collection from different parts of study area.

I acknowledge the financial assistance in the form of Wfsearch Eerrowship under project funded 6y the Department of Oceanic Development (DOD), Govt. of India, during my research career.

Einar& and especially, to my parents, my brothers and sisters, a very special thank for ad their rove, help and encouragement and for just being there.

Xs.hetrimayum Tomchou Singh.

CONTENTS

Page CONTENTS

LIST OF TABLES

LIST OF FIGURES vi

PREFACE

CHAPTER 1 INTRODUCTION

1.1 Introduction 1

1.2 Literature review 7

1.3 Scope of the study 14

1.4 Objective of the present study 17

1.5 Study area 17

1.4a Mandovi — Zuari estuarine system 17

1.4b Kalinadi Estuary 19

1.4c Aghanashini or Tadri Estuary 19

1.4d Kart Estuary 20

CHAPTER 2 MATERIALS AND METHODS 21 — 31

2.1 Introduction 21

2.2 Field methods 22

2.2a Sampling 22

2.2b Field observations 23

2.2c Sub sampling/ Storage 24

2.3 Laboratory methods 24

2.3a Sedimentological analysis 24

2.3a i. Sediment component analysis 24

2.3a ii. Clay mineralogy 25

2.3b Estimation of Organic Carbon (0C) 26

2.3c Geochemical analysis 26

2.3c i. Elemental analysis 26

2.3c ii. Radiochemical analysis 27

Page 2.3d Measurement of magnetic susceptibility 28

2.3d i. Sample preparation 28

2.3d ii. Measurements 29

2.3e Data processing 30

CHAPTER 3 SEDIMENT CHARACTERISTICS: MANDOVI —

ZUARI ESTUARINE SYSTEM 32 — 82

3.1 Introduction 32

3.2 Field observations 33

3.3 Sediment components 33

3.3a Results 33

3.3b Discussion 38

3.4 Clay mineralogy 43

3.4a Results 43

3.4b Discussion 45

3.5 Geochemistry 49

3.5a Results 49

3.5b Discussion 54

3.5b i. Vertical distribution 55

3.5b ii. Spatial distribution 60

3.6 Factor analysis 62

3.7 Sedimentation rate, Enrichment Factors and

Pollution history recorded in cores 66

3.8 Magnetic susceptibility 74

3.8a Results 74

3.8b Discussion 78

CHAPTER 4 SEDIMENT CHARACTERISTICS: KALINADI,

TADRI AND KARLI ESTUARIES 83 —146

4A KARWAR 83 —102

4A. 1 Field observations 83

4A. 2 Sediment components 83

4A. 2a Results 83

4A. 2b Discussion 87

Page

4A. 3 Clay mineralogy 88

4A. 4 Geochemistry 89

4A. 4a Results 89

4A. 4b Discussion 91

4A. 4b i. Vertical distribution 91 4A. 4b ii. Spatial distribution 94

4A. 5 Factor analysis 95

4A. 6 Sedimentation rate and Enrichment Factor 97

4A. 7 Magnetic susceptibility 98

4A. 7a Results 98

4A. 7b Discussion 101

4B GOKARN 103 —123

4B. 1 Field observations 103

4B. 2 Sediment components 103

4B. 2a Results 103

4B. 2b Discussion 105

4B. 3 Geochemistry 106

4B. 3a Results 106

4B. 3b Discussion 108

4B. 3b i. Vertical distribution 108 4B. 3b ii. Spatial distribution 111

4B. 4 Factor analysis 112

4B. 5 Sedimentation rate and Enrichment Factor 114

4B. 6 Magnetic susceptibility 117

4B. 6a Results 117

4B. 6b Discussion 120

4C MALVAN 124 — 146

4C. 1 Field observations 124

4C. 2 Sediment components 124

4B. 2a Results 124

4B. 2b Discussion 126

Page

4C. 3 Clay mineralogy 128

4C. 4 Geochemistry 130

4C. 4a Results 130

4C. 4b Discussion 132

4C. 4b i. Vertical distribution 132 4C. 4b ii. Spatial distribution 134

4C. 5 Factor analysis 135

4C. 6 Sedimentation rate and Enrichment Factor 136

4C. 7 Magnetic susceptibility 138 .

4C. 7a Results 138

4C. 7b Discussion 142

CHAPTER 5 REGIONAL SEDIMENTATION PATTERN 147 -157

5.1 Sedimentation rate 147

5.2 Sediment components and Geochemistry 151

4.3 Magnetic susceptibility 156

SUMMARY 158— 162

Recommendation for further work 162

REFERENCES 163 - 190

iv

LIST OF TABLES

Page

Table 2.1. Details of samples collected 23

Table 2.2. Pipetting time of 8D at different temperatures 25 Table 2.3. Magnetic properties and their interpretations 30

Table 3.1: Place of location of cores 32

Table 3.2. Change of sediment characteristics at different depths of

mudflats 39

Table3.3. Average values of sediment components and metals of cores

from different mudflats 61.

Table 3.4. Factor analysis matrix after varimax rotation showing correlations with principal components: cores MS and MR 63 Table 3.5. Factor analysis matrix after varimax rotation showing

correlations with principal components: cores ZA, ZR and CB 65 Table 4A.1. Clay mineralogy of the selected samples in core KH 88 Table 4A. 2. Factor analysis matrix after varimax rotation showing

correlations with principal components 96 Table 4B.1. Factor analysis matrix after varimax rotation showing

correlations with principal components 113 Table 4C.1. Factor analysis matrix after varimax rotation showing

correlations with principal components 135 Table 5.1. Sedimentation rates in different mudflats 148 Table 5.2. Average values of different sediment components and metals

in two phases of sedimentation 152

Table 5.3. Average values of sediment components and metals of cores

from different mudflats 152

Table 5.4. Regression coefficients (r) of sediment components and

metals in sediments 155

Table 5.5. Enrichment factors (EF) of elements in sediments from

different mudflats 154

LIST OF FIGURES

Page Fig.2.1. Flow chart of sediment sampling and analysis 21 Fig 2.2. Map showing the core locations along the study area 22 Fig 2.3. Photograph showing the collection of sample at Mandovi

Estuary 23

Fig 3. 1 Down core variation of sediment components: A — core

MS, B — core MR 34

C — core MB 35

Fig 3. 2. Down core variation of sediment components: A —core ZCH, B —core ZA, C —core ZC, D —core ZR 36 Fig 3. 3. Down core variation of sediment components: A — core

CA, B — core CB 38

Fig.3.4. Ternary diagrams for the textural classification of sediments on the basis of sand/mud ratios. A: after

Reineck and Siefert (1980). B: Flemming (2000) 40 Fig.3.5 Triangular diagram for the classification of hydrodynamic

conditions after Pejrup (1988): A — Mandovi, B — Zuari 41

C — Cumbharjua 42

Fig.3.6. Relative abundance (%) of clay minerals in the cores 43 Fig.3.7. Vertical distribution of clay minerals in the cores 44 Fig.3.8. Correlation between clay minerals (smectite, kaolinite),

clay fraction and organic carbon (OC) 48 Fig.3.9. Down core variation of metals in mudflat sediments of

Mandovi Estuary: A — core MS, B — core MR 50 Fig.3.10. Down core variation of metals in mudflat sediments of

Zuari Estuary: A — core ZA, B — core ZR 52 Fig.3.11. Down core variation of metals in mudflat sediments of

Cumbharjua canal (CB) 54

Fig.3.12. Isocon diagram. Individual points represent average value of sediment component and element in each core. To ensure all elements plot on the same scale, major elements (%): Al, K, Mg, Fe, sand, and OC are multiplied by 100; Ca and Mn (pg/g), by 1; Zn, Cu, Cr, Co, Pb, silt,

and clay by 10 61 vi

Page Fig.3.13. Down core profiles of 210Pb: A — core MS, B - core MR,

C — core ZA 67

Fig.3.14

Fig.3.15.

Down core trends of metal enrichment factors (EF): A —

core MS, B — core MR 69

C — core ZA, D — core ZR 70

E — core CB 71

Down core trends of metal sediment accumulation factors (Igeo): A — core MS, B — core MR, C — core ZA, D— core

ZR, E — core CB 73 Fig. 3.16. Magnetic profiles: A — core MS, B — core MR 74

Magnetic profiles of core ZA 76

Fig.3.17. Representative thermal demagnetization curves for different sections in cores MS and MR 77 Fig, 3.18. Representative curve of temperature dependence of

magnetic susceptibility 78

Fig. 4A. 1 Down core variation of sediment components: A — core

KH, B — core KM 84

Fig. 4A.2(A, B). Ternary diagrams for the textural classification of sediments on the basis of sand/mud ratios. A: after Reineck and Siefert (1980). B: Flemming (2000) 85 Fig. 4A.2C. Triangular diagram for the classification of hydrodynamic

conditions after Pejrup (1988) 86

Fig.4A.3. Down core variation of metals: A - core KH, B — core KM 90 Fig.4A.4. Isocon diagram. Individual points represent average value

of sediment component and element in each core. To ensure all elements plot on the same scale, major elements (%): Al, K, Mg, Fe and OC are multiplied by 100;

Ca, Cr and Mn (pg/g), by 1; Zn, Cu, Co, sand, silt, and

clay by 10 94 Fig. 4A.5. Down core profiles of 210Pb: A — core KH, B - core KM 97 Fig. 4A.6. Down core trends of metal enrichment factors (EF): A —

core KH, B — core KM 98

Fig. 4A.7A. Magnetic profiles of core KH 99

Fig. 4A.7B. Magnetic profiles of core KM 100

Page Fig.4A.8. Representative thermal demagnetization curves for

different sections of core KH and core KM 100 Fig.4B.1. Down core variation of sediment components: A — core

GH, B — core GN 104

Fig.4B.2(A, B). Ternary diagrams for the textural classification of sediments on the basis of sand/mud ratios. A: after Reineck and Siefert (1980). B: Flemming (2000) 105 Fig. 4B.2C. Triangular diagram for the classification of hydrodynamic

conditions after Pejrup (1988) 106

Fig. 4B.3. Down core variation of metals: A — core GH, B — core GN 107 Fig. 4B.4. Down core variation of clay minerals in core GN 109 Fig. 4B.5. Isocon diagram. Individual points represent average value

of sediment component and element in each core. To ensure all elements plot on the same scale, major elements (%): Al, K, Mg, Fe, sand, and OC are multiplied by 100; Ca and Mn (pg/g), by 1; Zn, Cu, Cr, Co, Pb, silt, and clay by 10 112 . Fig. 4B.6. Down core profiles of 210Pb: A — core GH, B - core GN 115 Fig. 4B.7. Down core trends of metal enrichment factors (EF): A —

core GH, B — core GN 116

Fig. 4B.8. Magnetic profiles: A — core GH, B — core GN 118 Fig.4B.9. Representative thermal demagnetization curves for

different sections in cores GH and GN 119 Fig. 4B.10. Representative curve of temperature dependence of

magnetic susceptibility 120

Fig 4C.1. Down core variation of sediment components: A — core

MAK, B — core MM, C —core MAJ 125

Fig.4C.2(A, B). Ternary diagrams for the textural classification of sediments on the basis of sand/mud ratios. A: after Reineck and Siefert (1980). B: Flemming (2000) 126 Fig. 4C.2C. Triangular diagram for the classification of hydrodynamic

conditions after Pejrup (1988) 127

Fig. 4C.3. Down core variation of clay minerals: A — core MAK, B -

core MM 128

viii

Page Fig. 4C.4. Down core variation of metals: A — core MAK, B — core

MAA 131

Fig. 4C.5.

Fig. 4C.6.

Fig. 4C.7.

Isocon diagram. Individual points represent average value of sediment component and element in each core. To ensure all elements plot on the same scale, major elements (%): Al, K, Mg, Fe, sand, and OC are multiplied by 100; Ca and Mn (pg/g), by 1; Zn, Cu, Cr, Co, Pb, silt,

and clay by 10 134, Down core profiles of 210Pb: A — core MAK, B - core MM 136 Down core trends of metal enrichment factors (EF): A —

core MAK, B — core MAA 138

Fig. 4C.8. Magnetic profiles: A — core MAK, B — core MM 139 Fig.4C.9. Representative thermal demagnetization curves for

different sections in cores GH and GN 141 Fig, 4C.10. Representative curve of temperature dependence of

magnetic susceptibility 142

Fig. 5.1. Variation of annual rainfall in Goa during last 100 years 149 Fig. 5.2. Global average sea level rise (after 4th report, IPPC, 2007) 150.

Fig. 5.3. Pie charts showing the variation of different clay minerals in sediments of different mudflats along study area 153 Fig. 5.4. Bar diagrams showing variation of different magnetic

parameters in sediments of mudflats along the study area 156

PREFACE

Intertidal mudflats are a prominent geomorphological component of estuaries and the development of an estuarine mudflat is both complex and difficult to predict because of the multiple relationship between the physical, chemical and biological properties of the sediment. The sediment distribution patterns within the mudflats are mainly controlled by river flows and also tides and are seemed suitable for studying environmental factors. One important component of the estuarine environment is the sediment, which acts as both a source and a sink for many contaminants, and supports a wide range of flora and fauna, which are important components of the aquatic food chain. Sedimentary sequences in estuaries and tidal marshes have been extensively studied elsewhere because of their demonstrative ability to preserve an undisturbed record of environmental change. A low rate of bioturbation coupled with a fairly high sedimentation rate provides an ideal substrate for the preservation and reconstruction of long-term and short-term changes in the coastal environment. Mudflats and saltmarshes are very sensitive to changes in sea level and the effects of reclamation and industry development.

Mature salt marshes thus tend to maintain their elevation relative to a tidal frame of reference, and vertical accretion may provide an indication of local sea level rise.

Estuarine and coastal areas are often regions of high population density and intense human activity. As a result, elevated levels of heavy metals in intertidl sediments, reflecting the impact of industrial development, have been well documented worldwide. Industrial and urban expansion around estuaries has commonly led to an increased input of heavy metals, a fraction of which sorb on to fine-grained suspended material and are subsequently deposited and buried in sub and intertidal mudflats and the saltmarshes. So, analysis of cores of undisturbed sediments allows reconstructing recent and historical inputs of metal contaminants.

Mudflats have been shown by numerous authors to be effective medium to long- term storage areas for a range of contaminants, although a detailed understanding of the specific controls on the trapping and storage of contaminants is absent.

The main objective of the present study is, to examine the small-scale (intra- estuary) and large-scale (inter estuary) spatial and temporal variation of sediment components, environmental status and the sedimentation rate and to understand

the possible processes that cause these variations. A broad range of mudflats from different estuaries within the study area has been covered. The central basis of this study is that the mudflats are developed in sheltered areas such as estuaries, lagoons and bays and despite strong hydrodynamics, intertidal estuarine mudflats are preferential sites for accumulation of fine-grained sediments, organic matter and metals originated from numerous marine and terrestrial sources including those of anthropogenic origin.

The thesis is presented in the following order:

Chapter 1 starts with general introduction and deals with the objectives of the present study by detailing the importance of the present study in conjunction with reliability and scope of the study, which lead to the birth of the objectives of the thesis. A detailed review of literature in relevant fields of the present study such as on general characteristics of sediment, post depositional movement of metals, construction of pollution history and sedimentation rate from global and Indian context is given, which is followed by the description of the study area.

Chapter 2 deals with the materials and methodologies used for collection and analysis of the sediment samples to obtain all necessary data for the characterization of sediments in order to meet the objectives of the proposed study.

In the present study, a total of 16 shallow cores with lengths varying from 38 cm to 92 cm were collected from different estuarine mudflats along central Western Coast of India. These includes Nine from Goa (Zuari and Mandovi Estuaries including Cumbharjua canal), Four from Karnataka (Karwar and Gokarn), and Three from Maharastra (Malvan). The sedimentological sand, silt and clay analysis was carried out by wet sieving and pipette method following Folk (1968); preparation of slides for clay minerals analysis by following the procedures of Rao and Rao (1995);

estimation of relative percentages of clay minerals by semi-quantitative method of Biscaye (1965) and estimation of Organic Carbon by wet oxidation method of Gaudette et al. (1974). The elemental analysis employed was an open wet digestion in Teflon beakers using a combination of HF, HCIO4 and HNO3 and the

analysis (210 Pb dating) was measured via its daughter nuclide 210Po following the standard radiochemical procedure of Flynn (1968). Analysis of different magnetic properties and computations are carried out by methods proposed by Thomson and Oldfield (1986), Maher (1988) and Oldfield (1990, 1994).

The results on sediment characteristics and discussions of the four areas of the present study are presented in two chapters namely Chapter 3: Mandovi — Zuari estuarine system and Chapter 4: Kali, Tadri and Karli estuaries. The results include distribution of sediments components, clay minerals, major and trace elements, evidence and effects of post depositional movement in metal distribution, rate of sedimentation, enrichment of metals with time and magnetic properties of sediments together with their implication on environmental study. Specific attention is given in chapter 3 to the mudflats of Mandovi Zuari estuarine system because this estuarine system is the largest along the study area and hence the mudflats are comparatively extensive, preserving the both natural and man-made changes in the catchment areas. The spatial variation of sediment characters and possible reasons in relation to abundance of sediment and chemical components within the individual estuary and creeks are also described in these chapters.

Chapter 5 describes the large-scale spatial or regional variation of sedimentation pattern with time in different mudflats of different estuaries and creeks along the study area. The variation in different parameters in relation with phases of sedimentation has been described in this chapter. The results of various parameters of sediments presented in chapters 3 and 4 are used to understand the variations in sediment source, environmental status and also the possible sources of pollution and factors responsible for sedimentation. In short, this chapter provides a synthesis and comprehensive assessment of the study area.

A summary of the work carried out and the salient features emanating from the interpretation and discussions of the study and also scope for future works is given separately.

The thesis is closed by a complete list of references cited in the text, tables and figures in alphabetical order.

xii

Chapter One

INTRoDvcTiow

1.1 Introduction

Intertidal zone is characterized by the presence of tidal flats, mudflats and salt marshes. Tidal flats and mudflats are inhabited by diverse community of organisms, whereas the salt marshes are vegetated by higher plants. Tidal flats occur in two types of settings. One is an exposed coast with low topography and relatively low tidal energy, and the other is a coastal region protected from the local high tidal energy such as an estuary, lagoon, bay, or other areas lying behind a barrier island.

In other words, the conditions necessary for the formation of a tidal flat include a measurable tidal range and absence of strong wave action. Intertidal deposits may consist of mineral grains from clay size to coarse sand, of carbonate particles of same size range, and of other organic materials such as Mollusk shells and shell fragments, coral debris, and plant remains. The sediment composition indicates the sediment that goes into the intertidal deposits may come from various sources, a nearby river, long shore transport from a source farther away, local or regional erosion or reworking of older deposits or biogenic source of locally or regionally produced carbonates.

Reineek (1972) defined tidal flats as "sandy to muddy or marshy flats emerging during low tide and submerging during high tide" and later, Klein (1985) defined tidal flats as "low relief environment containing unconsolidated and unvegetated sediments that accumulate within the intertidal range, including the supratidal zone". According to Bates and Jackson (1987), these are sandy-muddy depositional systems along marine and estuarine shores periodically submerged and exposed in the course of the rise and fall of the tide. They are coastal intertidal terrain with a unique character, including distinctive vegetation, and a variety of functions. They are sometimes freshwater, but often brackish or saline. These are the sites of sedimentary accretion where silts and other fine sediments accumulate in the shallow water of sheltered areas of bays, lagoons and estuaries or behind sand bars (Steers, 1969). So, tidal flats are a type of natural environment formed out of sand, silt and clay built up through the action of ocean tides and rivers and subjected to repeated changes of water level due to the ebb and flood tides. They develop where there is abundant sandy sediment on exposed coasts, sufficient to withstand the natural erosive forces. In more sheltered situations finer sediments of clays and silts settle out to form tidal mudflats.

Mud deposition is typically in protected low energy environments such as estuaries and lagoons. So, mudflats are found in sheltered areas such as estuaries, lagoons and bays. They occur in the lower intertidal zone where regular and increased depth of flooding prevents salt tolerant plants growing. Seaweed is absent because of the lack of a solid substrate. The shorelines of estuaries and coastal regions are often comprised of large areas of intertidal mudflats. Intertidal mudflats are a prominent geomorphological component of estuaries and the development of an estuarine mudflat is both complex and difficult to predict because of the multiple relationship between the physical, chemical and biological properties of the sediment (Patersion et al., 1990; Yallop et al., 1994). The largest mudflats occur in macrotidal (greater than 4 m) ranges. Intertidal mudflats represent large surface areas in macrotidal estuaries at low tide. Rapid and massive changes in the sediment level due to erosion, resuspension, advective transport and redeposition do occur in some locations specially in macrotidal estuarine mudflats, all these being controlled by waves and both tidal and river currents. Mudflats are not featureless, they are with ridges and mounds caused by differences in sediment texture and deposition rates and tidal channel networks. Signs of both erosion and deposition are often present on mudflats in close proximity. Despite strong hydrodynamics, intertidal estuarine mudflats are preferential sites for accumulation of fine grained sediments, organic matter and metals originated from numerous marine and terrestrial sources including those of anthropogenic origin.

Very fine grained sediments are transported by water in suspension. It may seem common sense to assume that silts and clays are deposited only when water bodies are sluggish and &ow. However, rivers and coastal waters are rarely slack enough for settling to occur by gravity alone. Mud is therefore deposited by the interaction of biological, physical and chemical processes. Organisms in the water column ingest microscopic particles of inorganic sediment as they filter the water for organic matter. These and other particles are excreted as sand sized pellets which sink more rapidly and accumulate on the bottom. Deposition is also enhanced by the presence of algal mats which form on muddy surfaces. Clay grains possess physical and chemical properties which make them bind together in saltwater, into aggregations which are large enough to sink through a process called 'flocculation'.

seawater and freshwater mix, by the presence of organic matter which also has strong chemical binding properties. An interesting feature of fine grained mud is due to their 'stickiness' they can resist being re-suspended and transported by moving water which would be capable of transporting larger, uncohesive sands and gravels.

On a global scale, tidal flat environments are exposed to two dominant tidal types, a semi- diurnal and a diurnal one which are separated by two mixed types consisting of a predominantly semi-diurnal and a predominantly diurnal one. Modern tidal flat depositional systems are commonly found in estuaries and along the low lying coastal plain shores between the equator and the polar seas at tidal ranges above about 0.5 m. They can attain shore-normal widths of several kilometers, alongshore extensions of many 10s to 100s of kilometers, and vertical accretion heights of several 10s to 100s of meters (Davis, 1994; Ehlers 1998). In Europe, almost all mudflats are in estuaries or tidal inlets, mainly tidal inlets, mainly because the exposure to waves on the open coast is too large to allow any significant amount of mud to accumulate. In other parts of the World, there are situations where open coast mudflats exist and these are usually associated with areas with a very large source of muddy sediments, for example the Chinese Coast around the mouths of the Yangtze and Yellow Rivers.

Tidal flats are excellent sedimentary environments for the preservation of trace fossils as well as physical sedimentary structures because the alternating layers of sand and mud enhance the expression of the structures. Tidal flats developed under regressive and prograding conditions are characterized by a fining upward sequence, consisting of coarse sediments at the base and progressively finer sediments toward the top in an uninterrupted, vertical sequence. This reflects the decreasing wave action in the progression from subtidal to intertidal to supratidal parts of the tidal flat. Commonly, this sequence is characterized by: (1) a dominantly sandy subtidal zone of channel fill, point bar and shoal sediments; (2) a mixed sand and mud intertidal flat deposit; and (3) a muddy upper intertidal flat, and (4) an algae flat or salt marsh deposit (supratidal flat). The intertidal flat displays a variety of intertidal sand and mud layers, including wavy, flaser and lenticular bedding. Wavy bedding is produced by irregular flow conditions, oscillating flow

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directions, or relatively high velocity flow. Flaser bedding occurs when thin streaks of mud are deposited between sets of cross-laminated sandy or silty sediment, indicating an environment where the flow has periods of current activity followed by periods of quiescence when mud is deposited. In contrast, lenticular bedding occurs when lenses of sand are preserved in mud, indicating an environment where mud is the favoured mode of deposition. The upper intertidal flat surfaces are at times bioturbated or contain slightly laminated mud with thin sand lamina. All of these structures, however, can be modified or completely destroyed by intense burrowing, bioturbation, or profuse tracks and trails.

The facies found in tidal flats are unsurpassed "sea level gauges", "tide gauges", and "climate recorders" (Hardie, 1977). An unambiguous record of the position of mean paleo-sea level is engraved within the intertidal subfacies of all ancient shallow marine carbonate deposits. The same intertidal subfacies carry a record, quantitatively determinable, of the tidal range in the depositional environment. At the same time, the subtidal subfacies record the ambient and storm wave energy levels across the ancient platform, and this information in turn reflects, at least in

a

qualitative way, the prevailing weather patterns. In addition, the nature of the supratidal subfacies is a direct response to the prevailing climate in the region (Hardie and Shinn, 1986).

Among its various attributes, the tidal marsh has been identified as a place of high productivity, a nutrient source, and a sink as well as a site for nutrient transformation. Compared with most other habitats, mudflats are relatively poorly researched and the processes occurring on them are not well understood. There is great diversity in the morphology, sediments, bed forms and ecology of mudflats, that relates to the changing balance of physical, sedimentological and biological forms on the sediments.

Estuarine sediments are an important sink for a wide range of contaminants, with heavy metals in particular showing a high affinity for fine grained estuarine sediments (Middleton and Grant, 1990; Cundy and Croudace, 1995a; Lee and Cundy, 2001; Spencer et al., 2003). Upon release into the aquatic environment,

subsequently gets removed from the water column and deposited in mudflat and salt marsh environments. Salt marshes in particular have been shown by numerous authors to be effective medium to longterm storage areas for a range of contaminants (Williams et al., 1994; Lee and Cundy, 2001), although a detailed understanding of the specific controls on the trapping and storage of contaminants is absent for many marsh/mudflat systems. Such an understanding is important, as, due to their potential role as contaminant storage areas, both salt marsh and mud flat environments may continue to release heavy metals into an estuary even after effluent discharge has ceased, due to a variety of physical, chemical and biological processes which may mix, remobilise and ultimately rework the metals into the water column through the processes of erosion, dredging, early diagenesis and bioturbation.

Vertical accretion of salt marshes occurs through the introduction of mineral material and through the accumulation of organic matter derived from net primary production of marsh vegetation (Redfield, 1972; De Laune et al., 1978). Within a European context, meso and macrotidal salt marshes with highly inorganic substrates are particularly important. Under normal conditions, salt marshes do not grow higher than the level of highest astronomical tide, since the frequency and duration of tidal flooding controls the introduction of mineralogenic sediments (Allen and Pye, 1992). Mature salt marshes thus tend to maintain their elevation relative to a tidal frame of reference, and vertical accretion may provide an indication of local sea level rise.

A tidal flat can be divided into sand flat and mud flat based on the components of sediments, and into the coastal tidal flat and the estuary tidal flat based on the location. The components of sediments are decided by the physical characteristics of individual area, and subsequently, greatly influence both the biological diversity and productivity of the habitat. The bottom of sand flat consists of sands of average size 0.2 - 0.7mm. The average sand flat width is 1 km, usually forming where the seawater flows quickly. The content of the organic matter is only 1 to 2 %. In contrast, a mud flat is formed where the stream of the seawater is gentle. The width of a mud flat averages over 5 km and is composed of particles of the average diameter of 0.031 mm. This tightly compacted sediment makes it harder for the

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seawater with oxygen and food to penetrate the flat. Tidal wetlands are one of the most productive ecosystems known. They produce large amounts of plant material, which are broken down into detritus by the myriad of animals and micro-organisms which live in the sediments of wetlands.

Tidal flats form at the edges of many tidal estuaries, and are found in three broad climatic regions (Dionne, 1988): (1) low-latitude tidal flats in arid and wet tropical or subtropical regions, where they may be colonized by mangroves; (2) mid-latitude tidal flats of temperate regions; and (3) high-latitude tidal flats influenced by ice.

They exhibit a wide variety of forms. The sediments are generally composed of mud and sands; the mud content, however, is sufficiently high for the sediment to exhibit cohesive properties. The mudflats can be bounded by sand flats near the low tide mark, and above high water neap tides by a zone of vegetation. Bordering temperate and sub polar estuaries this vegetation consists of halophyte plants that form special plant associations known as saltmarshes. In the tropics and sub- tropics salt marshes are replaced by mangrove communities. Mudflats can have distinct zonation of flora and fauna which can be extremely numerous and productive, though often of low diversity.

Intertidal mudflats can be separated into three distinct zones (Klein, 1985): the lower tidal flats lie between mean low water neap and mean low water spring tide levels and are often subjected to strong tidal currents; the middle flats are located between mean low water neaps and mean high water neaps; the upper flats lie between the mean high water neap and mean high water springs. The upper flats are the least inundated part of the mudflat and are only submerged at high water by spring tides. Salt marsh vegetation may colonize as far seaward as mean high water neaps. Mudflats will often continue below the level of low water spring tidei, and form sub-tidal mudflats (McCann, 1980). The upper flats are generally characterized by fine grained sediments, the middle flats by fine silts and the lower flats by sandy mud (Shi and Chen, 1996). As mentioned earlier, there is great diversity in the morphology, sediments, bedforms and ecology of mudflats. Intertidal areas are valuable ecological entities with a productive flora and fauna. They support large populations of birds, and form nursery and feeding areas for coastal fisheries. Mudflats and salt marshes are very sensitive to changes in sea level and

the effects of reclamation and industry. In order to predict the response of mudflats to environmental and anthropogenic pressures a greater understanding is needed that explains the observed differences between mudflats. A classification scheme would allow comparison between different mudflats and aid construction of a framework of general principles that will define the hierarchy of the underlying processes. The ability to predict the characteristics of estuarine mudflats will be of particular benefit to environmental managers. The first step was to construct a typology (Dyer, 1998) that describes the qualitative differences and similarities between mudflats, and groups them accordingly. Following this, a quantitative approach is necessary to develop a more rigorous classification. Though considerable work has been done on estuaries, in terms of their tidal range (Davies, 1964; Hayes, 1975), topography (Pritchard, 1952), physiographic features (Fairbridge, 1980), morphology (Dalrymple et al., 1992) and salinity structure (Pritchard, 1952; Cameron and Pritchard, 1963), little has been done to develop similar classification of the intertidal areas within estuary systems. McCann (1980) suggested four criteria for the classification of tidal flats: (1) sediment composition (carbonate or non-carbonate); (2) hydrographic position (intertidal or subtidal); (3) tidal range (macro, meso, micro); and (4) physiographic setting (estuary, delta, exposed coastline and continental shelf). He showed that tidal flats predominate in mesotidal and macrotidal settings with abundant sediment supply and low wave energy. Dionne (1988) suggested that tidal flats be classified on the basis of: (1) tidal range; (2) geomorphological setting; (3) sediment type; and (4) geographic location. Dyer (1998) developed a typology for temperate mudflats based on observations of the simplest set of diagnostic attributes, and the presence or absence of certain features. However, the limits of many of the categories awaited confirmation and quantification using numerical data.

1.2 Literature Review

Rudolf Richter's first exploration of the tidal flats of the German North Sea Coast, in the early 1920s marks the beginning of systematic research on tidal deposits and subsequently spread to Netherlands, England and France. Richter and his successors focused on the process of deposition, the sedimentary and organic structures and the fossils of the North Sea. The papers on the tidal flats can be found in, but not restricted to, the collective works of Thomson (1968) in the Gulf of

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California; Neumann et al. (1970) in the Caribbean Sea; Zhuang and Chappel (1991) in South Australia; Evans (1965) and Evans and Collins (1975) on the Wash; Van Straaten and Kuenen (1957) and Postma (1961) on the Wadden Sea;

Klein (1985) and Dalrymple et al (1990) on the Bay of Fundy; Champangne (1982), Anderson (1983), Grinham and Martini (1984), Dionne (1988) and Isla et al. (1991) in America etc. A considerable amount of information on the tidal flats around the Bohai and Yellow Sea has emerged that includes the works of Wang and Eisma (1988, 1990) in China; and that of Frey et al. (1989), Adams et al. (1990) in South Korea.

Recent research on tidal flats has altered in focus from studies of morphology and internal structure to measurements of tidal flat dynamics and environmental status.

Recently, Dyer et al. (2000) reported a classification scheme for intertidal mudflat sediments. Twenty attributes of 18 mudflats from North-West Europe have been analyzed statistically to establish a classification scheme. Correlation analysis, multidimensional scaling and cluster analysis have revealed five effective levels of mudflat classification. Deloffre et al. (2005) studied the hydrodynamic parameters that control the fluvial sediment dynamics on an intertidal mudflat located in a sheltered zone in the upper part (fluvial part) of the macrotidal Seine Estuary, France.

A considerable number of papers are available dealing with spatial and vertical variations and the processes that control the variation in sedimentological and geochemical parameters. The salt marsh/mudflat cores exhibit clear vertical zonation with distinct oxidizing and reducing conditions and the early diagenetic processes greatly modified the original distribution of metals (Cundy and Croudace,

1995b; Zwolsman et al., 1996; Spencer et al., 2003). Man et al. (2004) have reported the seasonal variation of the remobilization characteristics of these trace metals in the mudflat sediments. Of the four trace metals (Cd, Cr, Pb and Zn) cadmium showed the greatest tendency towards remobilization from the sediment phase to the more bio-available porewater phase. Zhanga et al. (2002) have studied the grain size effect on multi-element concentrations in sediments from the intertidal flats of Bohai Bay, China and found that most of the elements have their highest concentrations in the fine grained mud samples, in comparison with the silt

and silt-mud samples, with clay minerals possibly playing an important role. In contrast, concentrations of Ba, Sr, Hf, and Zr are elevated in the coarse silt samples. Ba and Sr may reside in feldspars, while Zr and Hf are present in zircons.

Landward to seaward spatial variation of element concentrations in the sediments was also reported with the spatial distribution of the grain size and is related to the seawater hydrological dynamics in the intertidal flats. Velde et al. (2003) have studied the contrasting trace element geochemistry in two American and French salt marshes to gauge how the different clay mineral sources and surface sedimentary diagenetic conditions affect the respective trace element geochemistry. These observations suggest that clay is a very important carrier in fixing trace elements for historical records in salt marsh sediments.

The tidal flat sediments data indicate a highly variable depositional regime in which organic matter is extensively degraded both before and after incorporation into the sediments. The organic matter content was strongly correlated with the abundance of the mud fraction, indicating the importance of organic matter sorption onto particles for preservation of both marine and terrestrial organic matter (Volkman et al., 2000).

Velde and Church (1999) have studied the important and quantitative changes in clay mineralogy which occur within a decade at the redox boundary in high marsh sediments near Lewes, Delaware. The clay mineralogy consists initially of a micaceous illite and chlorite mixture accumulating at the salt marsh surface. Once buried, these detrital clays are transformed into a new assemblage containing an illite/smectitic mixed layer mineral of poor crystallinity. On the other hand, Brockamp and Zuther (2004) have observed the changes in the clay mineral content of tidal flat sediments along the lower Saxony coast of the North Sea, Germany and concluded that they are mainly influenced by dike construction and sea level rise, modifying flow pattern, submarine morphology, sedimentation and tidal range.

Deloffre, et al. (2006) have analyzed the fine particles transfer (<63 pm) in the marine part of the man-altered macrotidal Seine Estuary, France in order to understand the controlling factors of rhythmic sedimentation processes on an intertidal estuarine mudflat. Again, Deloffre, et al. (2007) have investigated the formation of intertidal rhythmites, their preservation, and the evaluation of

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sedimentation rates on estuarine mudflats in the lower part of macrotidal estuaries (Medway Estuary, Kent, UK; Authie Estuary, France; Seine Estuary, Normandy, France). Quaresma et al. (2007) have investigated the sedimentary processes acting over an intertidal flat (Hythe, UK) located in Southampton Water, Hampshire, UK on the basis of hydrodynamic, morphological and sediment transport field measurements and concluded that the morphological characteristics of Hythe intertidal area appears to be controlled by a combination of prevailing hydrodynamic conditions and shell transport over the salt marsh.

Several studies have been carried out in salt marshes suggesting these areas may act as natural sinks for metals (Banus et al., 1975; Windon, 1975; McCaffrey and Thompson, 1980; Oenema et al., 1988; Grant and Middleton, 1990; Orson et al., 1992; Bricker, 1993; Williams et al., 1994; Otte et al., 1995; Gallagher et al., 1996;

Rosales Hez et al., 2003; Feng et al., 2004; Hwang, 2006). The importance of this storage ability comes, namely, from the fact that many salt marshes lie to heavily industrialized areas facing important discharges of metals and other toxic wastes.

Cundy et al. (2003) have reported the reliability of Pb-210 and Cs-137 dating, correlated with geochemical studies in reconstructing historical trends in heavy metal input and sediment accretion in heavily polluted estuarine areas in Europe(

Spain, UK, Sicily). Zhang et al. (2001) have stressed on importance of geomorphic influence on understanding of the heterogeneity of sediment grain size, which is vital to the assessment of pollution in intertidal sediments. Thomson et al (2002) have studied the records of radionuclide deposition in the two salt marshes in the United Kingdom with contrasting redox and accumulation conditions, while Rae and Allen (1993) have studied the significance of organic matter degradation in bringing about the early diagenetic mobility of anthropogenic trace metals and hence in historical trends of pollution. Jaffe et al. (2007) have studied the anthropogenic influence on sedimentation and intertidal mudflat change in San Pablo Bay, California and concluded that the timing and patterns of geomorphic change and deposition and erosion of sediment were influenced by human activities that altered sediment delivery from rivers. Cundy et al. (2005) have examined heavy metal distribution and accumulation in two contrasting Spartina sp.-dominated macrotidal salt marsh systems — a rapidly prograding, relatively young marsh system at the Vasiere Nord, near the mouth of the Seine Estuary, France, and a more mature,

less extensive marsh system in the Medway Estuary, UK., and reported that, overall, heavy metal concentrations at both sites are within typical ranges reported for other industrialized estuaries in NW Europe. Madsen et al. (2007) have investigated the temporal variability of estuarine sedimentation in the northernmost part of the Wadden Sea (Denmark), using an estuarine sedimentary sequence at Ho Havn and observed that the mudflats and salt marshes in the region show high accretion rates, indicating that the coastal lagoons could be less vulnerable and threatened by a future sea level rise than generally believed.

Studies related to the sedimentation rate, historical input and post depositional mobility of metals were carried out by Craft et al. (1993) in a Microtidal regularly and irregularly flooded estuarine marsh in North Carollina; Zwolsman et al. (1993) in Scheldt Estuary; Cundy and Croudace (1995b) on a series of salt marsh/

mudflats from the Hamble Estuary, Southern England and Bricker- Urso et al.

(1989) in Rhode Island Salt Marshes relationship with mean sea level. Ciavola et al.

(2002) have studied the sedimentation processes on Intertidal Areas of the Lagoon of Venice using profiles of natural 210 Pb (using the Constant Rate of Supply and Constant Initial Concentration) and found the sedimentation rates calculated for the two mudflats, Rosa and Saline, were more problematic to interpret because of downcore mixing and/or the occurrence of reducing conditions. On the other side, several papers have stressed on sedimentation, bioturbation and sediment contamination. Crusius et al. (2004) have studied the bioturbation depths, rates and processes in Massachusetts Bay. Sharma et al. (1987) worked on sedimentation and bioturbation in a salt marsh as revealed by Pb-210, Cs-137, and Be-7 studies.

De Laune et al. (1981) studied the accumulation of plant nutrients and heavy metals through sedimentation process in a Lousisna Salt marsh.

Many coastal marshes grow upward in response to rising sea level. For the past century, sea level has risen 10 - 20 cm (Gornitz et al., 1982; Barnette, 1983;

Revelle, 1983). Estimates of accretion in estuarine marshes of the North-Eastern (Redfield, 1972; Armentano and Woodwell, 1975; Bricker-Urso et al., 1989), Mid- Atlantic (Kearney and Ward, 1986; Griffin and Rabenhorst, 1989) and South- Eastern (Benninger and Chanton, 1985; Sharma et al., 1987) U.S.A. coast suggests that these wetlands are keeping pace with sea level rise.

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Of recent, researches on environmental magnetism related to intertidal flats have gained considerable attention. Environmental magnetism helps in investigating processes and their results in which magnetic particles will be transported, sedimented and resedimented, using rockmagnetic and palaeomagnetic methods.

Lecoanet et al. (2001) have analysed the magnetic properties of salt marsh soils contaminated by iron industry emissions in South-East France. Detailed magnetic properties of salt marsh soils exposed to intense atmospheric deposition of fly ashes from the iron industry are reported from South-East France. An enhancement in the concentration of magnetic particles in topsoil through this area was observed. Magnetic methods used to reflect not only the concentration of ferrimagnetic minerals but also their grain size, thus enabling discrimination of metallurgical dusts and fine pedogenic particles created in situ. Rubio et al. (2001) studied the sedimentological characteristics, heavy metal distribution and magnetic properties in subtidal sediments, Ria de Pontevedra, NW Spain. The depositional and postdepositional controls on magnetic signals were studied by Wheeler et al.

(1999) in salt marshes on North-West Coast of Ireland and by Zhang et al., (2001) in the intertidal sediments of the Yangtze Estuary, China. Berry and Plater (1998) have studied the rates of tidal sedimentation from records of industrial pollution and environmental magnetism in Tees Estuary, North-East England and concluded that in the absence of variation in grain size or post-depositional migration of sediment pollution, differences in the depth of synchronous peak metal and magnetic concentrations between cores can be attributed to the period and frequency of tidal inundation and, hence, accretion rate.

In overall, intertidal mudflats have been relatively poorly researched in comparison with sandy flats and salt marshes, though they are of great importance. They protect many kilometers of coastline by attenuating the waves, and are host to diverse and productive ecosystems that supports higher organisms including wading birds, shell-fish and fish stocks. There is very complex interaction and feedback between the physical, sedimentary, biological and chemical processes, which makes multidisciplinary measurements essential. This has prevented a clear understanding of the response of the form and function of mudflats to changing climate and sea level, and to anthropogenic inputs. There is not even a well

variety of mudflats, or to establish the hierarchy of processes causing those differences.

Research related to intertidal sediments in India is in embryonic stage. Very few works have been carried out along East Coast of India. Hameed et al. (2006) have studied the down core variations in organic matter, calcium carbonate content and sediment texture in five sediment cores collected from the tidal flats and estuaries between Cuddalore and Odinur, Tamil Nadu at —50-70 cm water depth. Data on sediment texture, mineralogy, organic matter content supplemented by radiocarbon dates indicates that the deposition of these sediments has taken place in phases since the early Holocene. Reworking of the inner shelf sediments as a result of the dropping sea level provided the ultimate sediment source for the progradation of the present coastline.

Ayyamperumal et al. (2006) have analysed the acid leachable trace metals in sediment cores from River Uppanar, Cuddalore, South-East Coast of India while Janaki-Raman et al. (2007) have reported that trace metals in sediments of Muthupet mangroves, South-East Coast of India are diagenetically modified and anthropogenic processes control Pb and to some extent, Ni, Zn and Fe.

Achyuthan and Baker (2002) have studied the geomorphology, clay mineral composition, and radiocarbon dates from Muttukadu to Marakkanam estuaries and the tidal zone along the East Coast of Tamil Nadu, India, and have attempted to reconstruct coastal evolution between approximately 4500 and 1100 BP. They have observed that Middle to Late Holocene coastal sedimentation and the chronology of the tidal zone formation have been strongly influenced by local factors. Chakrabarti (2005) has studied the sedimentary structures of some coastal tropical tidal flats of the East Coast of India, and inner estuarine tidal point bars located at 30 to 50 kilometers inland from the coast under varying seasonal conditions. Selvaraj et al.

(2003) have studied the geochemical processes controlling the distribution of nondetrital trace metals in sediment cores from Ennore Creek, South-East Coast of India.

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So, in Indian context, there is still paucity of information about these highly sensitive and important areas of interface between land and sea. As witnessed from available literature, work has not been carried out till date in the estuarine tidal mudflats especially along West Coast of India. However, these coastal environments are coming under increasing stress from human activity and the global climate change. There is increasing concern over the effects of anthropogenic inputs with the increase in population and associated rapid industrialization in and around the coast of India. Estuaries receive inputs from multiple sources as domestic and industrial sewage outfalls, effluents from mines, dredge spoil disposal and agricultural runoff and deposited in associated mudflats in turn, which are also witnessed in the Mandovi and Zuari estuarine system of Goa as well as in many other small rivers. In short, the estuarine mudflats are very sensitive to both manmade as well as natural environmental changes and preserve the characteristic changes that can be studied through sediments deposited with time. Therefore the present investigation will allow gaining insight in to the processes affecting the distribution of sediment components, the representation of pollution history and the concentration of heavy metals.

1.3 Scope of the Study

Sediments are the basis of the mudflat. They are transported to these mudflats as a result of erosion of adjacent shorelines and fine grains that make up the sediment load of the rivers. Sedimentary sequences in estuaries and tidal marshes have been extensively studied elsewhere because of their demonstrated ability to preserve an undisturbed record of environmental change. A low rate of bioturbation coupled with a fairly high sedimentation rate provides an ideal substrate for the preservation and reconstruction of long term and short term changes in the coastal environment. The sediment distribution patterns within the mudflats are mainly controlled by river flows and also tides. Therefore, mudflats are seemed suitable for studying broad relationship between environmental factors. More needs to be known about intertidal muds as integrators of overall processes and energy of sediment transport. In order to understand them insight, one need to improve the knowledge on the distribution, abundance, source and dynamics of their components. Most of the research works available on tidal flats are attributed to the

macrotidal estuaries. So, the first aim of the present study is to bring information on the mudflats of microtidal estuaries that was not attempted before.

Within the intertidal region of an estuary, sediments are composed of different geochemical phases that act as potential binding site for metals entering an estuarine system. These phases include clay, silt, sand, organic material, oxides of iron and manganese, aluminum and silica, carbonates and sulphide complexes (Shea, 1988). Of these components, oxides of iron and manganese and organic matter are considered the most important in influencing processes of metal transport, distribution and bioavailability (Louma and Bryan, 1982; Davis-Colley et al., 1984; Rule and Alden, 1996). In view of the role played by sediment matrix in either enhancing or reducing metal availability, an understanding of what factors contribute to this matrix would in turn allow us a greater predictive capacity of metal availability, their movement in these environments.

Spatial heterogeneity in the sediment geochemistry of an intertidal region has already been noted (Louma and Bryan, 1981; Langston, 1985; Morse et al., 1993).

However, processes that lead to this heterogeneity are yet to be well defined. In an estuarine environment two processes that can potentially contribute to the surficial sediment geochemistry include sediment diagenesis (Canfield, 1989) and riverine input (Benoit et al., 1994). The relative importance of these two processes contributing to the sediment matrix within an estuarine region is poorly known.

Hence, one of the primary objectives of the present study was to determine the relative importance of diagenetic processes vs. riverine input in contributing to sediment concentrations of oxides of iron and manganese, heavy metals throughout an estuarine intertidal region.

Tidal flats are semi-terrestrial habitats that interact with both aquatic and terrestrial environments. Runoff from upland areas contributes fresh water, detritus, nutrients and sediment and may be a source of pollutants such as herbicides, pesticides, fertilizers, industrial wastes and chemical spills. One important component of the marine environment is the sediment, which acts as both a source and a sink for many contaminants. There are five main sources of heavy metals in aquatic and sedimentary systems: erosion of geological sources, industrial processing of ores

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and metals, the use of metals and metal compounds in industry, the burning of fossil fuels, and leaching from refuse dumps (Wittmann and Forstner, 1980). The widespread use of heavy metals in industries ranging from large scale mining to intensive agriculture has resulted in a variety of heavy metals being released into the environment to concentrations in excess of the natural background levels. The potential environmental damage might be comparatively small if these metals are ultimately fixed in sediments and pore-water concentrations limited by their solubility (Dryssen and Wedborg, 1980). However, the decay of organic matter may enhance the harmful potential of the heavy metals as the ensuing reducing conditions can mobilize heavy metals, which become concentrated by physical or chemical processes. Heavy metals entering the aquatic and sedimentary environments can do so in a variety of chemical forms (De Groot et al., 1976).

Suspended matter tends to preferentially adsorb heavy metals in water and to provide the main mode of heavy metal emplacement in estuarine settings.

Estuarine and coastal areas are often regions of high population density and intense human activity. As a result, elevated levels of heavy metals in intertidal sediments, reflecting the impact of industrial development, have been well documented worldwide (Bryan and Langston, 1992; Ahn et al., 1995; Attrill and Thomes, 1995; Saiz-Salinas et al., 1996). Among the factors influencing the accumulation of heavy metals, particle size plays a significant role. Fine grained sediments often show higher concentrations of heavy metals due to their greater surface to volume ratio and enrichment of organic matter and Fe—Mn oxide (Williams et al., 1994; Rae, 1997). Since specific geomorphic units tend to have a particular sediment particle size distribution, an evaluation of the spatial variability of heavy metal concentrations must, therefore, take into account the influence of geomorphic variation (Fletcher et al., 1994; Ladd et al., 1998). Industrial and urban expansion around estuaries has commonly led to an increased input of heavy metals, a fraction of which sorbs on to fine grained suspended material and are subsequently deposited and buried in sub and intertidal mudflats and the saltmarshes. So, analysis of cores of undisturbed sediment may allow reconstructing recent and historical inputs of metal contaminants. The historical record of pollutant fluxes may also allow predictions of future concentration levels in the estuaries. With knowledge of past usage of the polluting substance and with a sense of potential changes in usage, we can in principle be in a position to foretell